This application claims the priority of Japanese application 8-238039, the disclosure of which is expressly incorporated by reference herein.
The present invention relates to a fuel pump, specifically to a fuel pump applicable for a high pressure fuel pump for the direct gas injection system for automotive gasoline engines.
In internal combustion engines, especially automotive gasoline engines, the direct gas injection system has been studied in recent years for the purpose of the improvement of fuel consumption performance, the reduction of harmful exhaust gas and the improvement of drive performance and acceleration performance.
In the direct gas injection system, in order to inject directly gasoline into the cylinders of the internal combustion engine even when the compression stroke, a high pressure fuel pump which can supply gasoline with the high pressure, for example, more than 3 MPa, is required.
In using gasoline for the lubrication of the drive part and the sealing of the rotor shaft of the fuel pump, as the viscosity of gasoline is extremely smaller than that of general purpose lubricating oil, the longevity of the rotational load support part of the drive part, especially the longevity of the bearing, becomes extremely short, and the reliability of the sealing mechanism of the rotor shaft is reduced.
In order to solve the above problem, as shown in Japanese Patent Laid Open No. 4-209981 (1992), a fuel pump is provided in which the pressure-up of the fluid can be facilitated easily by using secondary fluid having higher viscosity and lubrication properties, and the operation fluid can be pressurized up by the piston through the bellows, as well as the lubrication of the load support part such as bearing is performed by the secondary fluid, and the sealing mechanism of the rotor shaft can be established by the secondary fluid.
However, as only one piston is provided in the fuel pump described in Japanese Patent Application Laid-Open No. 4-209981 (1992), the pressure pulsation of the supplied fluid becomes larger. For the direct gas injection apparatus, it is required to establish the minimum fluctuation of the pressure of the supplied fuel with respect to the pressure control accuracy and response of the injected fuel and the flexible condition for the injection time selection. Although it is desirable to configure the piston multi-cylinder in order to reduce the pressure pulsation, the size of the fuel pump is larger in that configuration in which bellows are used for the individual piston as shown in Japanese Patent Application Laid-Open No. 4-209981 (1992).